Pat the roast dry. Rub with salt and pepper and place on an oiled fat pan of the oven. Cook in the preheated oven (electric cooker: 175 °C/ gas: level 2) for about 1 hour.
Peel, wash and chop the kohlrabi and carrots. Cook both in boiling salted water for about 10 minutes. Crumble the toast very finely. Pluck basil leaves from the stalks, wash and chop.
Mix toast, egg yolk, pesto and basil. Spread the roast 10 minutes before the end of cooking time. Clean, wash and chop the spring onions. Add to vegetables 2 minutes before the end of cooking time.
Drain the vegetables and collect the cooking water. Remove the roast from the fat pan and keep warm. Deglaze the roast with 1/4 litre of the vegetable water and pour through a sieve. Add cream and heat up. Melt cheese in it, bring to the boil and stir in sauce thickener.
Season the sauce with salt and pepper. Arrange roast and vegetables with some sauce on a plate. Remaining sauce extra is enough.